Electrochemical milling process

ABSTRACT

A PROCESS AND APPARATUS FOR THE SELECTIVE REMOVAL OF METAL FROM A CORROSION RESISTANT METAL PIECE BY ELECTROCHEMICAL MILLING BY COVERING THE PIECE WITH AN ALUMINUM OR TITANIUM ELECTRICALLY CONDUCTIVE MASK WHICH IS PROVIDED WITH A PROTECTIVE SURFACE THROUGH ANODIZATION DURING THE PROCESS, THE NON-MASKED, ETCHED SURFACES OF THE METAL PIECE BEING ELECTROCHEMICALLY DISSOLVED TO A PREDETERMINED DEPTH.

L Li John Zuryk Oct. 10, 1972 J. G. LUCAS EFAL 3,697,401

ELECTROCHEMICAL MILLING PROCESS Filed Jan. 9, 1970 2 Sheets-Sheet 1 I H/5 I I {I I H I FIG6Z.

INVENTORS Joseph 6. Lucas&

FIG.3. BY

ATTORNEY Oct. 10, 1972 ,1. G. LUCAS ETAL I 3, 91,401

ELECTROCHEMICAL MILLING PROCESS Filed Jan. 9, 1970 2 Sheets-Sheet 2FIG.4.

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nWENroRs Joseph G. Lucas 8 John Zuryk ATTORNEY United States Patent 01as Patented Oct. 10, 1972 3,697,401 ELECTROCHEMICAL MILLING PROCESSJoseph G. Lucas, Trumbull, and John Zuryk, Fairfield, Cnn., assignors toAvco Corporation, Stratford, Conn. Continuation-in-part of abandonedapplication Ser. No.

638,298, May 15, 1967. This application Jan. 9, 1970,

Ser. No. 1,830

Int. Cl. B23p 1/00 US. Cl. 204-143 R 9 Claims ABSTRACT OF THE DISCLOSUREA process and apparatus for the selective removal of metal from acorrosion resistant metal piece by electrochemical milling by coveringthe piece with an aluminum or titanium electrically conductive maskwhich is provided with a protective surface through anodization duringthe process, the non-masked, etched surfaces of the metal piece beingelectrochemically dissolved to a predetermined depth.

This application is a continuation-in-part of application Ser. No.638,298 filed May 15, 1967 and assigned to the same assignee, nowabandoned.

This invention is directed to a novel process and apparatus forelectrochemical milling of workpieces, particularly articles of aferrous nature. Actually, the process is most adaptable to theelectrochemical milling of corrosive resistant ferrous alloys asstainless steels, commonly used in the manufacture of such commercialequipment as heat exchangers and like apparatus, wherein milling isdesired within fine tolerances to provide internal cavities ordepressions of predetermined size and shape.

The process may be briefly summarized as one wherein the corrosionresistant workpiece is initially masked prior to submersion in theelectrolyte by a valve metal mask or overlay having cut out portionstherein representing the area in the piece to be milled. The essence ofthe invention is that the same electrolytic action which serves to millthe workpiece renders the mask resistant, so that only the unmasked areais milled.

It is well known that certain valve metals, including aluminum andtitanium, are subject to an anodic oxidation, or become anodized whensubjected to certain acidic reactants. The term anodized is herein usedto characterize the hard, noncorroding oxide film which is deposited onthe surface of such metals. The term valve metal as used herein isdefined as including any metal having characteristics such that themetal becomes anodized when immersed in an acidic electrolyte throughwhich current is passed and wherein the oxide of said metal resultingfrom such oxidation is substantially nonconductive and passivated withrespect to the electrolytic action. Such metals include aluminum andtitanium. Apparently because of this characteristic of these metals inparticular, it is characteristic of the art that such elements or thealloys thereof have been discounted as being useful as anodes inelectrolytic baths, particularly where the electrolyte is one such assulphuric acid, for such an anode immediately becomes anozided and itssurface conductive capacity, as an anode, thereby immediatelypassivated, immunized or destroyed. However, in the instant inventionadvantage is taken of this phenomenon by utilizing a mask made of valvemetal and held in tight surface to surface and fluid excluding contactwith the workpiece with resultant eflicient current flow through theinterior of both mask and piece, whereas the exposed surface of themask, in practically, immediately becoming anodized when immersed in theelectrolyte, becomes immune to attack by the electrolytic bath. Theresult is twofold: Etlicient current flow through mask and piece, asindicated above, and secondly, confinement of electrolytic conduction,electrolysis or ion travel to the precise areas desired, and withgreater concentration or elfect within such areas where milling to beachieved. As stated, under these circumstances the workpiece itselffunctions as the anode.

The process of our invention, extremely efiicient in the commercialpractice thereof, is considerably less expensive than either themechanical or machining method of milling, or mere chemical milling withits comparatively costly masking methods and as well, complex chemicalsolutions necessitated by such a practice. The process and maskingmaterials employed in accordance with this invention contemplateutilization of materials which by their very nature result in adissolving effect on the exposed surfaces of the pieces (here corrosionresistant steel) and a passivity or anodic film formation on the maskingfixture whether it be an aluminum, titanium alloy or any other valvemetal. Thus the fixtured part is subjected to the electrochemical actionof a typical sulphuric acid anodizing bath under those conditions ofvoltage, temperature and solution makeup which are usually practiced inthe anodizing art. At any rate, during such process the steel piece isselectively dissolved away to the desired depth while the rack orfixture, performing both as an electrical conductor and as a preformedmask, is itself protected from metal loss by the formation of thereferred to anodic coating over its exposed surface.

It is, accordingly, a primary objective of our invention to provide anelectrochemical milling process wherein valve metals such as aluminum ortitanium, or alloys thereof, are utilized as masking or overlaymaterials, these forming the conductor to the anode, but at the sametime these, at the very inception of the milling procedure beingimmediately anodized upon their exterior or exposed surfaces by actionof the electrolyte so that not only is full current flow to those areasto be milled obtained but, also, the protective film of oxideresultantly formed upon the exposed surfaces of such aluminum ortitanium overlay has the effect of concentration of the current orconcentration of ion fiow to those areas in the piece directly exposedto the electrolyte and sought to be electrochemically milled.

It is an additional object of the invention to provide a mask for use inelectrochmical milling of the type herein described wherein, either byuse of a heavy metal mask of aluminum or titanium, or a coating of thesurface of the workpiece with such a metal or alloys thereof,substantial undercutting is eliminated in the sense that the opening oropenings in the mask representing the portions of the workpiece to bemilled substantially parallel, in diameter size, the size of the milledarea.

It is a further object of the invention to provide a preformed, rigidmask of substantial thickness so mounted with regard to the workpiece tobe milled that there is little or no leakage of electrolyte between maskand workpiece. Any slight leakage that occurs results in the immediateanodization of the mask at the leakage area with the resultant oxideformation rendering any area between piece and mask inactive insofar asany electrochemical milling therein be concerned; the practical resultis that this natural effect reduces or eliminates any substantialundercut.

It is another object of the invention to provide a process ofelectrochemical milling wherein the overlay or mask takes the form of adeposited surface of aluminum or titanium upon the workpiece. The latteris of minimal thickness, resulting in an even sharper definition of themilled area with even lesser undercutting thereof. In such practice thedeposited film of aluminum or titanium can be easily removed, aftermilling, by subjection to strong basic or acidic reactants.

A further objective of the invention is the provision of a process forelectro milling wherein a combination mask may be utilized; in thisregard a preformed, rigid mask comprised of a relatively thick valvemetal, such as aluminum or titanium, can surmount an underlying layer ofdeposited material of the same characteristic, this combinationresulting in an even greater lessening of undercutting in the milledarea:

Other objects and advantages of the instant process will be understoodby those skilled in the art from consideration of the following moredetailed description thereof. In connection therewith, the attachedfigures demonstrate a means for practicing the inventive processutilizing the so-called heavy or permanent type of mask. In thesefigures:

FIG. 1 is a top plan view of a solid mask superimposed over the materialto'be milled, which latter material is securely fastened in between themask and a lower imperforate piece of the same or similar material, theworkpiece being so clamped between same as to be in solid contact withthe aluminum or titanium overlay, the latter thereby conducting currentfrom an outside source through the interior thereof and to the piece tobe milled, which piece in this arrangement represents the anode;

FIG.2 is a section view taken on the line 2-2 of FIG. 1;

FIG. FIG. 1;

FIG. 4 is a top plan view of a workpiece which has been milled throughthe practice of our process, indicating the configuration cut thereinand, also, the slight areas adjacent the area which has been milledwhere some leakage of electrolyte has occurred but where no milling hasoccurred because .of the anodizing action in such areas;

FIG. 5 is a section view taken on line 5-5 of FIG. 4, showing, inexaggerated form, the depth of the milling cut and, also, the absence ofsubstantial undercutting at the edges thereof; and

FIG. ,6 illustrates diagrammatically the nature of a socalled undercutor inwardly beveled edge which is undesired and which is substantiallyprevented by the practice of the instant process.

FIG. 1 represents an assembly useful for accomplishing a method of theinvention wherein the overlay 5 consists of an aluminum or titaniumpiece having cut therein, as at 6, an opening or pattern of theconfiguration desired to be milled in the workpiece. The edges of thisopening 6 are beveled, as at 7, and within prescribed ranges ofangularity, as will hereinafter be described. The piece to be milled isindicated at 15 and it is placed in between the overlay 5 and anopposite plate 10 which may or may not be of the same material as theoverlay so long as it is passive toward the electrolyte. This back plate10, however, is preferably of an exterior configuration to match theexterior rectangular configuration of the overlay 5.

The workpiece is thus compressed between these two members 5 and 10, thelatter being held together in more or less,1 permanent fashion by boltswith an intermediate shim or gasket 18 spaced therebetween, thus leavingan interior space 19 for insertion of the part to be milled. Such member18 is preferably of the same metal as the overlay but may also be of amaterial passive to the electrolyte. It may have cut therein aconfiguration more or less matching the exterior configuration of theworkpiece. The space 19 provided by the shim 18 sandwiched between 5 and10 is thus such that it will permit insertion and, removal of theworkpiece, the latter in this embodiment of the invention having theconfiguration shown in dotted line in FIG. 1. As stated, gasket 18 maybe configured to complement the external V-shape of the piece 15 as itis' here shown. Thus space 19 will permit ready insertion of theworkpiece prior to milling and removal therefrom after milling.

These ,two plates 5 (the overlay) and underlying member 10 are furtherprecisely located with regard to each 3 is a section view taken on theline 33 of .4 other by a series of pins 25 positioned in appropriateapertures in elements 5, 18 and 10, respectively, so that the parts ofthe assembly prior to immersion in the electrolytic bath are accuratelyand precisely located with respect to each other in the manner shown.

A close fitting in face to face relationship between element 5,workpiece 15 and lower plate 10 is desired, this in order that anysubstantial leakage of electrolyte is prohibited, the electrolyticaction thus being confined only to the area outlined by theconfiguration 6 in the overlay. To this end, suitable bolts 40 and 42are so located as to exert pressure upon the underside of the workpiece,and as indicated in FIG. 2. These, when threaded towards the part to bemilled will exert a pressure thereupon. To further insure adequatepressure and completely tight face to face contact between workpiece andoverlay, an additional center bolt 45 is employed, this again, when suchis taken up, compressing the two elements 5 and 10 together with theworkpiece 15 positioned therebetween.

A suitable container for the electrolytic bath, made of material immuneto electrolytic action, is diagrammatically indicated in dotted line at30. The assembly as heretofore described is now placed in the bath andsubjected to the electrochemical milling action. Sulphuric acid, in thecase of employing either an aluminum or titanium mask or overlay ispreferably used as the electrolytic solution to mill the involvedferrous material, as stainless steel, to the pattern represented by theconfiguration 6. A voltage from a source V is fed to the aluminumoverlay 5 through line 50, the latter being firmly connected to a bolt22, threaded directly into the aluminum or titanium overlay 5. Thevoltage source is grounded as at G and, of course, the metal containerfor the bath, 30, is similarly grounded, as at G lWith this arrangement,current fiow is through the interior of the overlay 5, through theworkpiece 15 because of the face to face contact with the overlay,through the milling bath and thence to the ground G The preferredconcentration of the. sulphuric acid bath is 30% sulphuric acid byweight, although the same can be varied from 10 to 40% by weight, with apreferred range of concentration being from 20 to 30% by weight.Amperage values should be about 200 to 400 amperes. Such 200 to 400amperes current can be defined as being in the range of from about 5 to25 amperes/ink Variations within such ranges will accommodate thoseparameters dependent upon overall time of milling, characteristics ofmilled piece, depth of cut desired, et cetera.

iR6llaI1CC is placed upon the fact that valve metals such as aluminumand titanium, when placed in a sulphuric acid bath, become anodized uponthe surfaces exposed to the bath, this causing such surfaces to becoated with aluminum or titanium oxide, as the case may be. The anodizedsurface is not only completely passive insofar as any acidic attack beconcerned, but also renders the valve metal non-conducting at thoseanodized surfaces.

The result is that the current in the bath is concentrated within thoseareas desired to be electrochemically milled, and in the embodiment ofthe invention herein described, within that area defined by theconfiguration 6 of FIG. 1.

In this arrangement, the overlay or mask 5 may be considered merely asthe conductor for the electrical current, the workpiece itselfcomprising the anode for ionic transfer. It is apprehended that for thisvery reason, the two named metals have not been considered adaptable foruse as anodes in any electrochemical milling procedure or proceduresequivalent thereto-this simply because once exposed to acidic attackduring electrolysis, the resulting and practically instantaneousanodizing of the surface of these metals renders them completely passiveinsofar as current conductivity be concerned; in the instant case thatproblem has been resolved, and with the abovenamed advantages, byutilizing the mask as merely a conductor for the current to the anode(the workpiece) with the exposed portions of that conductor being notonly passivated or immune to current flow, but also immune to attack bythe acidic dielectric.

The metal to metal contact between workpiece and overlay eifectuatessuch a seal that leakage therebetween is practically eliminated.Furthermore, whatever small amount of leakage that does occur is of noconsequence and because of these factors: The stippled area which isdesignated at 36 in FIG. 4 indicates a slight discoloration or someeffect of bath contact between the surfaces of overlay and workpiece,and particularly at those edges thereof which terminate together andwhich are exposed to the electrolyte. These areas are exaggerated simplyfor illustrative purposes. They represent areas which have been slightlyanodized by slight leakage of the electrolyte between overlay andworkpiece. The resultant chemical reaction which takes place at suchpoints results in the formation of the oxide on the surface, as A1 inthe case of aluminum, or TiO in the case of titanium. In other words,this is the anodizing effect, resultant upon the entrapped electrolyteafter the first surge of electric current. The fine layer of oxide thereformed prevents any milling activity at these points; hence, suchleakage presents no problem insofar as obtaining a clean and reasonablysharp milling operation at the intersecting edges of the workpiece andoverlay. Stated somewhat differently, a resultant higher electricalresistance in the thin film of leaked and spent electrolyte plus therelatively longer electrical path from the cathode to the shielded anodelimit the current density to a value which is insignificant whencompared with the conditions prevailing on the front or selectivelyexposed side of the panel being milled.

There are, of course, two reactions which take place in each instancewhere either aluminum or titanium is employed as a metal for theoverlay. The first of these reactions involves the anodizing of theexposed surfaces of the overlay. In electrolysis this, of course, shouldbe represented in ionic form. The following equation typicallyrepresents the formation of the oxide (A1 0 on the surface:

This anodizing or oxidation of the surface of either the aluminum ortitanium takes place in the first few seconds of operation; and in thatlength of time such exposed surfaces become passivated for the remainderof the operation.

The second result of electrolysis is, of course, to perform the millingoperation wherein the ferrous alloy is milled to the desired depth. Inionic form this removal of metal during electrolysis in the sulphuricacid bath may take the following form, and assuming the stainless steelpiece be considered, for the sake of simplicity, merely as the metaliron:

The equations for anodization of a titanium mask and metal removal byelectrochemical milling when this metal be used are the same or similar.

When a preformed, permanent and reusable mask or overlay of aluminum ortitanium is used, the angle of inclination of the side of the maskrepresenting the areas to be milled, is of some significance. The edgeor side here referred to is that designated by the numeral 7 in FIGS. 1and 2. The referred to angularity is for the purpose of serving twobasic functions: Firstly, such a predetermined angularity has beenselected primarily to aid in the discharge of gas (oxygen) and partiallyspent electrolyte from the plate being milled; and, secondly, suchpredetermined angularity reduces the amount of possible undercutting toan extent where such undercutting (and as represented in FIG. 6) doesnot constitute a factor of appreciable importance. This preferredangularity may vary from between 30 to 45 to the vertical, as such side7 is viewed, e.g., in FIG. 2. When the angularity is of lesser orgreater amounts to the vertical, more than an acceptable amount ofundercutting, so-called, may ensue. For practical purposes, then, therange of 30 to 45 is preferred. The following table indicates the amountof undercut which occurs at 45 and the amount at 30, properinterpolation varying that amount between these two angularities:

This amount of undercut, of course, represents, in the case of a 45angle, the difference between .475 and .406 of an inch and with regardto a 30 angle, the difference between .475 and .406. By interpolation itcan be assumed, if the angle be 37- /2, the average of these two wouldresult in .0715 inch of undercut. When depth of removal of the metal isanywhere from between about .135 to about .175 inches, as would becommon in practical applications of the process, this amount ofundercutting is insignificant.

Mention in the foregoing has been made of an alternate practice of theinventive process wherein, instead of employing a heavy, reusableoverlay of aluminum or titanium, one or the other of such metals can bedeposited upon the workpiece by such methods as vapor deposition, etcetera. When so deposited they take the configuration of the portion ofthe workpiece to be milled. In this instance, undercutting can bereduced somewhat. In one example of the invention, a piece of stainlesssteel was overlaid with aluminum by dipping the workpiece in moltenaluminum. In an additional practice of the invention, a steel panel waspainted with a silicone type aluminum bath, heated to about 750 F. toremove most of the vehicle or carrier, leaving only the aluminumoverlay. Gas or vapor plating a workpiece with aluminum can also providethe overlay. The aluminum coated workpiece, however the coating may beapplied, after cathodic or chemical or mechanical removal of thealuminum from the desired geometric pattern, when subsequently anodical-1y, electrochemically exposed, showed the depth and pattern typical ofthe mechanical mask except that the entry edge of the cavity wassomewhat sharper.

Subsequent removal of the aluminum or titanium when such is applied bythe methods just referred to can be accomplished by simple chemicaldissolution in either a strongly alkaline or acid solution whichpreferentially dissolves the aluminum. In this instance, either sodiumhydroxide or potassium hydroxide are representative of strong alkalines.As an acidic solution to obtain aluminum removal, hydrofluoric acid isuseful for this purpose.

In the following table, similar to Table I, the amount of undercuttingis indicated where a vapor deposited, or hot metal dipped or paintedoverlay of aluminum has been utilized:

If advantages of both systems be desired, then the mechanical mask oroverlay can be used in conjunction with an undercoat of, e.g., aluminum,applied to the workpiece surface by any of the methods hereinbeforeindicated. In sucheventuality the amount of undercut would representthat shown in Table II, where advantage has been taken of. the sharpercut due to the minimal thickness of the overlay when such systems asvapor deposition are utilized for application thereof to the workpiecesurface.

It has been further found that when an applied aluminum overlay (asdistinguished from the permanent mechanical preformed type) is used inthe process, the overlay is still adequate to effectively conductcurrent at the given amperage values from an outside source to the workpiece, the anode.

Other advantages of the inventive process, particularly with regard tothe method involving a permanent, reusable aluminum ,or titaniumoverlay, should be apparent to thoseskilled in the art.

.Because made of a hard and durable metal, a preformed mask can bereused many times. When a good design is desired, it can be'more easilycut into such a heavy metal base rather than into such materials asLucite, or

similar resinous materials which tend to shatter or splinter even duringthe most precise cutting operations. Also, the assembly of the mask withthe workpiece, as a mechanical matter, is very simplified and can bedone manually in a few minutes of time, this also being true ofdisassembly thereof'after milling has been achieved. The operationitself is of relatively short duration-the cut or milled area in a pieceto a depth of about one-tenth of an inch can be achieved in about .10minutes and, of course, where multiple etchings are desired in a seriesof the same elements all placed in the same bath, this same period oftime is involved.

As indicated in the foregoing, and whether aluminum or titanium be theselected metal as the maskant and conductor to the workpiece, theprocess is effective when using sulphuric acid in the concentrationsheretofore given as the electrolyte. Commercially this represents adesirable feature of the process because of the relatively low cost ofthis inorganic acid in bulk quantities.

While the present invention has been illustrated and described in theforegoing with reference to certain particular embodiments thereof, itis not intended that it be limited to same, nor otherwise than by theterms of the claims appended hereto.

We claim:

1. An electrochemical milling process for the milling of a ferrous basemetal piece comprising masking said metal piece with a removable maskingoverlay of a valve metal, said valve metal having characteristics suchthat it becomes anodized when immersed in an acidic electrolyte throughwhich a current is passed, and wherein the oxide of said metal resultingfrom said anodizing is substantially nonconductive, said overlay havingan opening therein representing the shape to be milled, said overlaybeing in sealed face-to-face relationship with said piece, immersingsaid piece and said overlay in an acidic electrolyte, passing currentthrough said overlay, said piece and said bath, said workpiece being theanode, whereby said overlay becomes anodized at the exposed surfacesthereof and electrochemical milling action is confined to said shape insaid piece, and thereafter removing said removable masking overlay.

2. The invention as defined in claim 1 wherein said overlay metal isessentially aluminum and said acidic electrolyte is a solution ofsulphuric acid.

3. The invention as defined in claim 2 wherein the concentration of saidsolution is from 10 to 40 by weight.

4. The invention as defined in claim 3 wherein the current density isfrom about 5 to about 25 amperes/infi.

5. The invention as defined in claim 1 wherein said overlay opening iscircumscribed by edges having an angle to a line normal to the surfaceof said overlay of from about 30 to about 45 whereby undercutting duringsaid milling process is substantially reduced.

6. The invention as defined in claim 1 wherein said overlay metal isessentially titanium.

7. The invention as defined in claim 6wherein said acidic electrolyte issulphuric acid in a concentration of from about 10 to about 40 byweight.

8. The invention as defined in claim 7 wherein said overlay opening iscircumscribed by edges having an angle to a line normal to the surfaceof said overlay of from about 30 to about 45 whereby undercutting duringsaid milling process is substantially reduced.

9. The invention as defined in claim 8 wherein the current density isfrom about 5 to about 25 amperes/inF.

References Cited UNITED STATES PATENTS 1,376,365 4/ 1921 Wertheimer204--143 2,846,346 8/ 1958 Bradley 148-33 3,058,895 10/1962 Williams204-143 3,192,135 6/ 1965 Robbins 204-11 3,197,391 7/1965 Bowers 204-333,278,411 10/ 1966 Williams 204-290 3,294,653 12/1966 Keller et al204-58 3,522,492 8/ 1970 Pierce 317-235 2,186,721 1/ 1940 Guild 204-35 NFOREIGN PATENTS 821,115 9/1959 Great Britain 204--38 A 1,009,518 11/1965 Great Britain 204-143 TA-HSUNG TUNG, Primary Examiner US. Cl. X.R.204-15, 58

